1. Field of the Invention
The present invention relates generally to the packaging of semiconductor integrated circuits and more particularly to "leadframes" that provide electrical connection between a packaged integrated circuit and the external environment.
2. Background and Related Art
Semiconductor integrated circuits (ICs) are often housed or encapsulated in plastic packages in order to protect them from damage or contamination. Typically, a packaged IC includes a number of electrically conductive leads which provide a means of electrical connection between the encapsulated IC and the external environment. The IC packaging, including the leads, also facilitates the placement and attachment of the IC on printed circuit boards (PCBs), also known as printed wiring boards (PWBs).
Current IC packaging or "assembly" processes typically include some or all of the following steps:
1. Form leadframe PA1 2. Attach IC die to leadframe bonding pad PA1 3. Locate leads PA1 4. Bond connection wires to IC and leads PA1 5. Encapsulate in plastic epoxy (mold) PA1 6. Debar and dejunk PA1 7. Deflash PA1 8. Plate Outer leads PA1 9. Trim/Form PA1 10. Rework (if necessary)
While actual packaging processes often include other steps and substeps, brief consideration of the foregoing steps is sufficient to illustrate certain significant drawbacks and shortcomings of the current processes.
A significant number of the assembly steps, and particularly those concerned with the formation and connection of the leads, require expensive equipment, tooling, materials, personnel, space and utilities. In fact, current packaging processes may often account for as much of the cost of the finished product as the IC itself. In current high leadcount IC packages, such as a 208 pin quad flat pack (QFP), the lead related steps alone typically account for 20-25% of the total packaging cost.
Further, while some of the materials and designs of the subcomponents used in the packaging process represent true value additions relative to circuit functionality, protection and performance, others serve principally as process aids which are either incorporated in the package or which must subsequently be dealt with or removed at considerable additional expense and with attendant reductions in yield. A significant example is the dambar structure typically used in current leadframe designs primarily to facilitate the plastic molding or encapsulation process and secondarily to provide additional support for the leads. During the encapsulation process, the dambar is supposed to function as a clamping surface for the edges of the mold and as a barrier to prevent leakage or "flashing" of the epoxy from the mold onto the leads. Unfortunately, however, even with the high clampdown forces typically used, a completely uniform seal is not always formed between the dambar and the edges of the mold. One reason is that several strips of leadframes are typically processed simultaneously in each molding operation. Variations between strips can cause discontinuities in the seal between the gasket and the mold edges. Additionally, epoxy particulates can build up or be deposited on the mold clampdown surfaces and prevent formation of a complete seal. It is thus not at all uncommon for epoxy to "streak" out from the mold cavity in those regions where the seal is incomplete, resulting in "flashing" on the outer leads.
Following encapsulation, not only must the dambar be removed to physically and electrically individualize the leads, but in addition the "flashing" must be removed prior to subsequent plating of the leads, to assure complete and uniform coverage of the plating material. Accordingly, additional "debar" and "dejunk," or alternatively dambar removal and "deflash," are required. It is a significant drawback of current leadframe designs and of the packaging processes that use such designs that the debar, dejunk and deflash operations (1) are required at all, (2) are relatively expensive, (3) are time consuming and (4) adversely effect yield.
For example, the dambar is typically removed mechanically using a fairly high tonnage punch press. Typically, for higher lead count packages, a two stage tooling assembly is used. For a 208 lead package, for example, the tool will have 208 punches. However for certain configuration having dual dambars and inner and outer leads, such as a molded carrier ring (MCR) configuration, twice as many punches are required. For example, for a 208 lead MCR package, the tool will have 416 punches. In the first stage, half the punches excise the portions of the dambar(s) between every second lead. In the second stage, the remaining punches excise the remaining portions of the dambar(s).
The typical cost for tooling today to debar a 208 I/O leadframe with a pitch of .gtoreq.0.025 inch is approximately $100,000. For a 208 lead MCR package, the cost of tooling is significantly higher. As the outer lead pitch is decreased to 0.020 inch or less, the cost of tooling increases still more dramatically.
In addition, during the debar operation, epoxy trapped in the dam can easily crack and shatter. Fissures are thus created within which solder may deposit between the leads during subsequent plating operations. This can lead to shorting of the leads or at least compromise in electrical performance. Moreover, stray pieces of epoxy can become lodged on the punches causing damage and/or requiring reworking of the tooling. Including reworks, the typical punch usually has a useful "life" of approximately 750,000 "hits." Obviously, in order to maintain continuous operation a second tool is required while one is being reworked or otherwise serviced. The requirement for a second tool means additional capital investment attributable to the dambar removal process. Additionally it is both time consuming and expensive to rework the punch tools. Moreover, as the space between leads (pitch) changes with different leadframe designs, new tooling and hence a further investment of capital are required.
In addition to the direct costs associated with tooling, the typical debar process can result in yield losses due to offsets between the tools and the leads. Such offsets result in the unintended removal of material from the leads as well as the dambar, which may unacceptably weaken or even sever a lead. Such offsets arise from variations in the tooling (locating) holes on the leadframe as well as from tooling misalignments, for example. Losses at this nearly final stage of the semiconductor manufacturing process are particularly expensive. Even offsets that do not result in immediate rejection during the debar/dejunk operation may give rise to lead skewing and/or deviation from co-planarity of the outer leads in the subsequent trim/form operation. Parts that exhibit these problems have to be reworked manually so that they will properly fit on a PCB during any subsequent attachment operation, for example a solder reflow operation. Obviously such manual rework is both time consuming and expensive as well.
The debar operation can also adversely impact subsequent steps of the packaging process, for example the plating and trim/form operations. In the trim/form operation, for example, the outer portions of the leads are trimmed and the leads formed into the familiar "gull wing" configuration. The cutting of the dambar can result in rough edges in certain areas on the leads. During the plating process, the areas containing these rough edges tend to draw more current, and hence to accumulate more plating material, often in the form of "balls" or nodules. These nodules can transfer onto the tooling used in the trim/form operation and wear and contaminate the tooling, which in turn require more frequent cleaning and maintenance.
Following the debar operation, a "deflash" operation is required to remove any epoxy that may have "flashed" onto the outer leads during the encapsulation operation. The deflash operation also serves to remove any debris generated during the debar operation. Although the deflash operation generally does not result in significant yield losses per se, it does exhibit some very undesirable side effects. For example, the media (slurry or dry) or the chemical used to deflash may have the undesired side effect of destroying the "skin" of the molded package while removing the unwanted "flash" from the leads. The "skin" normally functions as a pseudo- "moisture barrier" to a substructure that is quite porous. The removal of the skin therefore exposes the substructure and renders it more susceptible to chemical ingress during the various chemical treatments of the subsequent plating process. In addition, the deflash operation, whether "media" or chemical, requires the disposal of used media/plastic and/or solvents. Of course, in addition to the disposal costs, there is a cost for the "media" and/or the solvents themselves. In the case of solvents, there are also in-plant costs for containment and significant safety concerns.
From the foregoing it can be seen that there is a significant need for improvements in the structure of leadframes and the IC packaging operations associated therewith in order to reduce costs, improve yields, improve throughput and eliminate various technical disadvantages. Eliminating some or all of the dambar related operations would significantly reduce the capital investment, the operating costs of packaging ICs, eliminate yield losses associated with these process steps and decrease production time.
Hence, it is an object of the present invention to provide an improved leadframe structure which overcomes the various drawbacks and disadvantages of known leadframe structures and the attendant drawbacks and disadvantages of conventional IC packaging processes that use such leadframes.
It is another object of the present invention to provide an improved leadframe structure which will significantly reduce the size of the IC assembly operation, reduce capital and operating costs and improve manufacturing yields.
It is a further object of the invention to provide an improved leadframe structure which eliminates the dambar structure of known leadframe designs.
It is a still further object of the invention to provide an improved leadframe design which reduces or eliminates flashing during the epoxy molding process and which reduces or eliminates the need for conventional debar, dejunk and deflash operations in the packaging of ICs.
It is a still further object of the invention to provide an improved leadframe design which facilitates the manufacturing of very fine outer lead pitch leadframes without requiring a lateral decrease in lead width, as is often associated with photochemical (etching) manufacturing processes.